The present invention relates generally to computerized image processing, and more specifically to techniques for locating a particular portion of an image and extracting mark-sense information therefrom.
Forms with mark-sense fields (such as check boxes) are ubiquitous, and provide an effective way to gather and communicate information. For certain forms, such as those used for college and professional school admission tests, the instructions for making a mark in a box are very explicit, the conditions are tightly controlled, and the people making the marks are highly motivated to do so correctly. The result is that the forms can be machine-read very accurately. In other contexts, the people filling in the check boxes are likely to have neither the correct No. 2 pencil nor the patience to ensure that they mark the box in an optimal manner. Fortunately, the consequences of having the form misread by a computer tend to be lower in such contexts. Indeed, many such forms require handwritten alphanumeric information and are read by people rather than machines.
One context that presents some unique challenges to computerized image processing is a form that has been faxed once or twice. Consider a scenario where a computer generates a form and transmits it by a fax modem over the telephone line to a remote fax machine, and where a user receives the paper copy of the form from the fax machine, scribbles entries on the form (including checking certain boxes), and transmits the form to the computer from the remote fax machine. On the basis of which boxes have been checked, the computer is supposed to take appropriate action. The form is assumed to have registration marks (fiducials), with the boxes at known locations relative to the fiducials.
In general, since the form was generated by the computer and did not have to be scanned into a fax machine at the computer site, the user at the remote fax machine will receive a form of reasonably high quality. The form received by the user may, however, contain pepper noise (isolated black pixels in white regions, or vice versa) or thin horizontal lines of black pixels in white regions, or vice versa, due to noise on the telephone line. By the time the originating computer receives the filled-in form, the image is likely to have been further degraded in one or more of the above ways and one or more of the following additional ways. The image is subject to scanner noise which can manifest itself as vertical black streaks due to opaque material on the scanning platen of the fax machine. Additionally, misregistration of the check box relative to the fiducials can occur because scan lines in the fax are lost as a result of transmission noise or because the image has become distorted due to imperfections in the mechanical feed. If the feed causes the input page to be skewed, the result is trapezoidal distortion. Moreover, one or more of the registration fiducials can even be lost due to a page fold during scanning or as a result of large skew.